1. Field of the Invention
The present invention relates to: a frequency modulation circuit to be used for communication devices such as mobile phones and wireless LAN devices; a transmission circuit; and a communication device. The present invention particularly relates to: a frequency modulation circuit which is capable of outputting a frequency-modulated signal with a high precision regardless of a modulation method to be applied; a transmission circuit using the frequency modulation circuit; and a communication device using the frequency modulation circuit.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to secure precision of a transmission signal and operate with low power consumption. Such communication devices are required to have a frequency modulation circuit for outputting a frequency-modulated signal with a high precision and a transmission circuit using the frequency modulation circuit. Hereinafter, a conventional frequency modulation circuit will be described.
FIG. 8 is a block diagram showing an exemplary structure of a conventional frequency modulation circuit 500. As shown in FIG. 8, the conventional frequency modulation circuit 500 comprises a comparing section 501, loop filter 502, VCO (voltage-controlled oscillating circuit) 503 and a feedback section 504. The VOC 503 comprises a VOC core 5031 and a variable capacitor 5032. As an input signal, a modulation signal is inputted to the frequency modulation circuit 500 via an input terminal. In the frequency modulation circuit 500, the modulation signal, which is inputted via the input terminal, branches so as to be inputted to the VOC 503 through two paths. The modulation signal on one path is inputted to the VOC 503 via the comparing section 501 and the loop filter 502. The modulation signal on the other path is inputted to the VOC 503 directly from the input terminal.
The VOC 503 controls an oscillatory frequency thereof in accordance with the modulation signal which is inputted via the loop filter 502 or directly from the input terminal, thereby frequency-modulating the modulation signal, and then outputs a frequency-modulated signal. To be specific, the VOC 503 controls, based on the inputted modulation signal, a voltage applied to the variable capacitor 5032, thereby controlling the oscillatory frequency of the VOC core 5031.
The loop filter 502 suppresses high-frequency components of the modulation signal inputted via the comparing section 501, and outputs the modulation signal. The feedback section 504 feeds, back to the comparing section 501, the frequency-modulated signal outputted from the VOC 503. The comparing section 501 compares a frequency of the inputted modulation signal with a frequency of the frequency-modulated signal outputted from the VOC 503, and causes the frequencies to synchronize with each other, thereby stabilizing the frequency of the frequency-modulated signal outputted from the VOC 503.
The feedback section 504, comparing section 501 and loop filter 502 form a feedback loop, whereas the path connecting from the input terminal to the VOC 503 forms an open loop. Generally speaking, the frequency modulation circuit 500 realizes highly precise frequency modulation by using the feedback loop, and realizes wideband frequency modulation by using the open loop.
However, the conventional frequency modulation circuit 500 has a problem in that when the circuit 500 is applied in various modulation methods, highly precise low-noise frequency modulation is not always realized. For example, it is conceivable that the frequency modulation circuit 500 is applied in a modulation method used for a relatively narrow band, such as GMSK or EDGE (hereinafter, referred to as a narrowband modulation method), or in a modulation method used for a relatively wide band, such as UTMS (hereinafter, referred to as a wideband modulation method). For example, in the case where the frequency modulation circuit 500 is applied in a wideband modulation method, frequency modulation is required to be realized in a band which is dozens of times wider than in the case where the circuit 500 is applied in a narrowband modulation method.
To be specific, in the case where the frequency modulation circuit 500 is applied in a wideband modulation method, the modulation signal to be inputted to the variable capacitor 5032 has a far greater amplitude than in the case where the frequency modulation circuit 500 is applied in the narrowband modulation method. Since the variable capacitor 5032 is designed to accept the modulation signal having such a great amplitude, when, in the case where the frequency modulation circuit 500 is applied in a narrowband modulation method, the modulation signal having a small amplitude is inputted to the variable capacitor 5032, a signal-to-noise ratio (SN ratio) of the modulation signal significantly deteriorates due to noise influence. Thus, there is a problem that the conventional frequency modulation circuit 500 is not always able to realize highly precise low-noise frequency modulation.